19,230 research outputs found
A survey of unified constitutive theories
The state of the art of time temperature dependent elastic viscoplastic constitutive theories which are based on the unified approach werre assessed. This class of constitutive theories is characterized by the use of kinetic equations and internal variables with appropriate evolutionary equations for treating all aspects of inelastic deformation including plasticity, creep, and stress relaxation. More than 10 such unified theories which are shown to satisfy the uniqueness and stability criteria imposed by Drucker's postulate and Ponter's inequalities are identified. The theories are compared for the types of flow law, kinetic equation, evolutionary equation of the internal variables, and treatment of temperature dependence. The similarities and differences of these theories are outlined in terms of mathematical formulations and illustrated by comparisons of theoretical calculations with experimental results which include monotonic stress-strain curves, cyclic hysteresis loops, creep and stress relaxation rates, and thermomechanical loops. Numerical methods used for integrating these stiff time temperature dependent constitutive equations are reviewed
Unified constitutive models for high-temperature structural applications
Unified constitutive models are characterized by the use of a single inelastic strain rate term for treating all aspects of inelastic deformation, including plasticity, creep, and stress relaxation under monotonic or cyclic loading. The structure of this class of constitutive theory pertinent for high temperature structural applications is first outlined and discussed. The effectiveness of the unified approach for representing high temperature deformation of Ni-base alloys is then evaluated by extensive comparison of experimental data and predictions of the Bodner-Partom and the Walker models. The use of the unified approach for hot section structural component analyses is demonstrated by applying the Walker model in finite element analyses of a benchmark notch problem and a turbine blade problem
Rheumatic conditions in human immunodeficiency virus infection
Many rheumatic diseases have been observed in HIV-infected persons. We, therefore, conducted a comprehensive literature search in order to review the prevalence, presentation and pathogenesis of rheumatic manifestations in HIV-infected subjects. Articular conditions (arthralgia, arthritis and SpAs) are either caused by the HIV infection itself, triggered by adaptive changes in the immune system, or secondary to microbial infections. Muscular symptoms may result from rhabdomyolysis, myositis or from side-effects of highly active anti-retroviral therapy (HAART). Osseous complications include osteonecrosis, osteoporosis and osteomyelitis. Some conditions such as the diffuse infiltrative lymphocytosis syndrome and sarcoidosis affect multiple organ systems. SLE may be observed but may be difficult to differentiate from HIV infection. Some anti-retroviral agents can precipitate hyperuricaemia and are associated with arthralgia. When indicated, immunosuppressants and even anti-TNF-α agents can be used in the carefully monitored HIV patient. Thus, rheumatic diseases and asymptomatic immune phenomena remain prevalent in HIV-infected persons even after the widespread implementation of highly active anti-retroviral therap
The night-sky at the Calar Alto Observatory
We present a characterization of the main properties of the night-sky at the
Calar Alto observatory for the time period between 2004 and 2007. We use
optical spectrophotometric data, photometric calibrated images taken in
moonless observing periods, together with the observing conditions regularly
monitored at the observatory, such as atmospheric extinction and seeing. We
derive, for the first time, the typical moonless night-sky optical spectrum for
the observatory. The spectrum shows a strong contamination by different
pollution lines, in particular from Mercury lines, which contribution to the
sky-brightness in the different bands is of the order of ~0.09 mag, ~0.16 mag
and ~0.10 mag in B, V and R respectively. The zenith-corrected values of the
moonless night-sky surface brightness are 22.39, 22.86, 22.01, 21.36 and 19.25
mag arcsec^-2 in U, B, V, R and I, which indicates that Calar Alto is a
particularly dark site for optical observations up to the I-band. The fraction
of astronomical useful nights at the observatory is ~70%, with a ~30% of
photometric nights. The typical extinction at the observatory is k_V~0.15 mag
in the Winter season, with little dispersion. In summer the extinction has a
wider range of values, although it does not reach the extreme peaks observed at
other sites. The median seeing for the last two years (2005-6) was ~0.90",
being smaller in the Summer (~0.87") than in the Winter (~0.96"). We conclude
in general that after 26 years of operations Calar Alto is still a good
astronomical site, being a natural candidate for future large aperture optical
telescopes.Comment: 16 pages, 5 figures, accepted for publishing in the Publications of
Astronomical Society of the Pacific (PASP
New intensity and visibility aspects of a double loop neutron interferometer
Various phase shifters and absorbers can be put into the arms of a double
loop neutron interferometer. The mean intensity levels of the forward and
diffracted beams behind an empty four plate interferometer of this type have
been calculated. It is shown that the intensities in the forward and diffracted
direction can be made equal using certain absorbers. In this case the
interferometer can be regarded as a 50/50 beam splitter. Furthermore the
visibilities of single and double loop interferometers are compared to each
other by varying the transmission in the first loop using different absorbers.
It can be shown that the visibility becomes exactly 1 using a phase shifter in
the second loop. In this case the phase shifter in the second loop must be
strongly correlated to the transmission coefficient of the absorber in the
first loop. Using such a device homodyne-like measurements of very weak signals
should become possible.Comment: 12 pages, 9 figures, accepted for publication in the Journal of
Optics B - Quantum and Semiclassical Optic
In-medium broadening of nucleon resonances
We analyze the effects of an in-medium broadening of nucleon resonances on
the exclusive photoproduction of mesons on nuclei as well as on the total
photoabsorption cross sections in a transport calculation. We show that the
resonance widths observed in semi-inclusive photoproduction on nuclei are
insensitive to an in-medium broadening of nucleon resonances. This is due to a
simple effect: the sizeable width of the nuclear surface and Fermi motion.Comment: 4 pages, 3 figures, minor changes in the tex
Phase-Sensitive Tetracrystal Pairing-Symmetry Measurements and Broken Time-Reversal Symmetry States of High Tc Superconductors
A detailed analysis of the symmetric tetracrystal geometry used in
phase-sensitive pairing symmetry experiments on high Tc superconductors is
carried out for both bulk and surface time-reversal symmetry-breaking states,
such as the d+id' and d+is states. The results depend critically on the
substrate geometry. In the general case, for the bulk d+id' (or d+is) state,
the measured flux quantization should in general not be too different from that
obtained in the pure d-wave case, provided |d'| << |d| (or |s| << |d|).
However, in one particular high symmetry geometry, the d+id' state gives
results that allow it to be distinguished from the pure d and the d + is
states. Results are also given for the cases where surface d+is or d+id' states
occur at a [110] surface of a bulk d-wave superconductor. Remarkably, in the
highest symmetry geometry, a number of the broken time-reversal symmetry states
discussed above give flux quantization conditions usually associated with
states not having broken time- reversal symmetry.Comment: 6 page
Nanoscale addressing and manipulation of neutral atoms using electromagnetically induced transparency
We propose to integrate dark-state based localization techniques into a
neutral atom quantum computing architecture and numerically investigate two
specific schemes. The first scheme implements state-selective projective
measurement by scattering photons from a specific qubit with very little cross
talk on the other atoms in the ensemble. The second scheme performs a
single-qubit phase gate on the target atom with an incoherent spontaneous
emission probability as low as 0.01. Our numerical simulations in rubidium (Rb)
atoms show that for both of these schemes a spatial resolution at the level of
tens of nanometers using near-infrared light can be achieved with
experimentally realistic parameters
Detection of pathological high-frequency oscillations in refractory epilepsy patients undergoing simultaneous stereo-electroencephalography and magnetoencephalography
BACKGROUND: Stereo-electroencephalography (SEEG) and magnetoencephalography (MEG) have generally been used independently as part of the pre-surgical evaluation of drug-resistant epilepsy (DRE) patients. However, the possibility of simultaneously employing these recording techniques to determine whether MEG has the potential of offering the same information as SEEG less invasively, or whether it could offer a greater spatial indication of the epileptogenic zone (EZ) to aid surgical planning, has not been previously evaluated. METHODS: Data from 24 paediatric and adult DRE patients, undergoing simultaneous SEEG and MEG as part of their pre-surgical evaluation, was analysed employing manual and automated high-frequency oscillations (HFOs) detection, and spectral and source localisation analyses. RESULTS: Twelve patients (50%) were included in the analysis (4 males; mean age=25.08 years) and showed interictal SEEG and MEG HFOs. HFOs detection was concordant between the two recording modalities, but SEEG displayed higher ability of differentiating between deep and superficial epileptogenic sources. Automated HFO detector in MEG recordings was validated against the manual MEG detection method. Spectral analysis revealed that SEEG and MEG detect distinct epileptic events. The EZ was well correlated with the simultaneously recorded data in 50% patients, while 25% patients displayed poor correlation or discordance. CONCLUSIONS: MEG recordings can detect HFOs, and simultaneous use of SEEG and MEG HFO identification facilitates EZ localisation during the presurgical planning stage for DRE patients. Further studies are necessary to validate these findings and support the translation of automated HFO detectors into routine clinical practice
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